1. Field of the Invention
The present invention relates to an antenna device and a radar apparatus.
2. Description of the Related Art
In radar systems, an array antenna forms a beam to transmit a signal. Then, the array antenna receives an echo signal which is corresponded to the signal in order to measure a target angle. The array antenna includes several subarray antennas and feeding interfaces. Each subarray antenna includes antenna elements and feed lines. The feed lines feed signals to the antenna elements. The antenna element may be any one of a patch, horn, slot antenna. The feed line may be any one of a microstrip line, a triplate line, a waveguide, a dielectric waveguide, and a post-wall waveguide. A waveguide feeding is popular for the antenna in automotive radar systems using the millimeter wave.
One of the waveguide feedings is disclosed in Japanese Patent No. 3843946. In this reference, several waveguide are formed by piling two metal walls. These waveguide provide a choke structure in order to avoid coupling of high frequency signals current in the waveguides.
In the automotive radar systems, interval between the adjacent subarray antennas should be narrow in order to achieve a wide coverage angle. For example, the interval between the adjacent subarray antennas is better to be equal to or smaller than 0.6λ, where λ is a free-space wavelength, in order to achieve the coverage angle of 40-degree.
However, in the case of waveguide feeding, it is difficult to achieve the narrow interval between the adjacent subarray antennas, when all feeding interfaces are formed at the same side of all subarray antennas. Moreover, extra space is required between adjacent subarray antennas in order to keep high isolation between the feeding interfaces.
As a result, the interval between the adjacent subarray antennas becomes wider and size of the antenna device becomes larger.